Allow reserved_ memmap to be invalid for heap test

In some cases the mapping fails (presumably due to ASLR). Avoid
testing it's valid to prevent test flakiness.

Test: test.py
Bug: 175018342
Change-Id: Ia177a62ffa2158e1c3e93d30e71f35fb3dc73158
diff --git a/runtime/gc/heap_test.cc b/runtime/gc/heap_test.cc
index 817c876..5e8c1e3 100644
--- a/runtime/gc/heap_test.cc
+++ b/runtime/gc/heap_test.cc
@@ -42,7 +42,7 @@
                                      /*reuse=*/ false,
                                      /*reservation=*/ nullptr,
                                      &error_msg);
-    ASSERT_TRUE(reserved_.IsValid()) << error_msg;
+    // There is no guarantee that reserved_ will be valid (due to ASLR). See b/175018342.
     CommonRuntimeTest::SetUp();
   }